1. Field of the Invention
The present invention relates to a tuning-fork piezoelectric vibrating reed, a piezoelectric vibrator having the piezoelectric vibrating reed, and an oscillator, an electronic device, and a wave clock each having the piezoelectric vibrator, as well as a manufacturing method of the piezoelectric vibrating reed.
2. Background Art
Recently, a piezoelectric vibrator utilizing quartz or the like is used in a cell-phone and a personal digital assistance as the time source, the timing source of a control signal, a reference signal source, and so forth. The piezoelectric vibrator of this type is proposed in a variety of forms, and a type having a tuning-fork piezoelectric vibrating reed is one example.
A tuning-fork piezoelectric vibrating reed includes a pair of vibrating arms disposed parallel to each other, a base portion to which a pair of the vibrating arms is integrally fixed at the base end side thereof, a pair of exciter electrodes formed on the outer surfaces of a pair of the vibrating arms to vibrate a pair of the vibrating arms, and mount electrodes electrically connected to a pair of the exciter electrodes, respectively. A pair of the exciter electrodes is an electrode that vibrates a pair of the vibrating arms at a specific resonance frequency in a direction to come close to or move apart from each other when a predetermined voltage is applied thereon via the mount electrodes. Both a pair of the exciter electrodes and the mount electrodes are electrode films formed by laminating a base metal layer and a finishing metal layer.
Incidentally, as is represented by a cell-phone, various electronic devices enclosing the piezoelectric vibrator are becoming smaller in recent years. Accordingly, there is a need for the piezoelectric vibrator to further reduce the size. Naturally, there is a need of a further size reduction for the piezoelectric vibrating reed forming the piezoelectric vibrator. Meanwhile, when the piezoelectric vibrating reed is reduced in size, it is general that the length of the vibrating arms becomes shorter.
The tuning-fork piezoelectric vibrating reed has a characteristic that the vibration characteristic deteriorates as the vibrating arms becomes shorter. A size reduction of the piezoelectric vibrating reed therefore gives rise to an inconvenience that the R1 value (serial resonance resistance value) becomes higher. In particular, because the R1 value is proportional to the effective resistance value Re, when the R1 value becomes higher, there arises a problem that it becomes difficult to operate the piezoelectric vibrating reed on low power.
When the tuning-fork piezoelectric vibrating reed is reduced in size, for example, groove portions are formed on the both principal surfaces of a pair of the vibrating arms along the longitudinal direction of the vibrating arms in order to lower the R1 value. For instance, WO00/44092 is an example of this art. The cross section of a piezoelectric vibrating reed provided with groove portions is shown in FIG. 26. As is shown in FIG. 26, a piezoelectric vibrating reed 200 has a pair of vibrating arms 202 each provided with groove portions 201 on the both principal surfaces. Owing to the groove portions 201, a pair of the vibrating arms 202 is formed in such a manner that each has a cross section in the shape of a letter H. Also, a pair of exciter electrodes 203 and 204 having different polarities is formed on the outer surfaces of a pair of the vibrating arms 202 by laminating a base metal layer 205 and a finishing metal layer 206.
According to the piezoelectric vibrating reed 200 configured as above, in particular, because the groove portions 201 are formed in the vibrating arms 202, a pair of the exciter electrodes 203 and 204 is in a positional relation to oppose each other to the extent possible. Hence, in comparison with a case where no groove portions 201 are formed, the electric field can act more efficiently. Accordingly, the vibration characteristic is improved because a vibration loss is lessened, and the R1 value can be as low as possible.
In order to achieve a size reduction of the piezoelectric vibrating reed while suppressing an increase of the R1 value, providing the groove portions as described above is effective.
As has been described, the electrode film forming a pair of the exciter electrodes 203 and 204 and the like is generally formed of two layers including the base metal layer 205 and the finishing metal layer 206. Normally, the base metal layer 205 is made of metal having a high hardness, such as chromium, in order to protect the piezoelectric vibrating reed 200. Meanwhile, the finishing metal layer 206 is made of soft metal having a low electric resistance value, such as gold, in order to enhance the ease of mounting and the conductivity with the lead terminals. It is general to use two kinds of metals made of different materials for the base metal layer 205 and the finishing metal layer 206 to suit different purposes.
Incidentally, the piezoelectric vibrating reed described above has the following problems that remain to be solved.
As has been described, it is effective to provide the groove portions in order to suppress the R1 value that increases with a size reduction of the piezoelectric vibrating reed. However, there is a processing limitation in forming a groove portion in the vibrating arm that has been made smaller. More specifically, because the vibrating arms themselves become smaller due to a size reduction, it is extremely difficult to form the groove portions in the vibrating arms that have been made smaller. Hence, there is a case that the piezoelectric vibrating reed fails to ensure a desired R1 value.
Under these circumstances, in order to ensure a constant R1 value, the size of the piezoelectric vibrating reed has to be increased up to a certain point.